Test device and method for simulating migration and transformation of nitrogen in underground water of river bank undercurrent zone

Abstract

The invention relates to a test device and method for simulating migration and transformation of nitrogen in underground water of a river bank subsurface zone, and the device comprises a detachable organic glass tank, the internal space of which is divided into a left-end water tank, a middle sand tank and a right-end water tank from left to right in sequence, and the left-end water tank, the middle sand tank and the right-end water tank are respectively used for simulating a river channel, a river bank and a far-bank aquifer; the water level control assembly comprises two constant water head control modules and platform lifting modules corresponding to the two constant water head control modules; the pore water level monitoring assembly comprises a plurality of piezometric tubes, a plurality of transparent graduated scales and a photographing module; the water quality sampling assembly comprises a pore water filtering module embedded in the middle sand tank in an array manner, a vacuum pumping module and a water sample collecting module connected with the pore water filtering module and the vacuum pumping module. Compared with the prior art, the sampling time of the pore water of the fine-particle soil is greatly shortened, and the accuracy of a test result is ensured; automatic control, whole-process monitoring and rapid and automatic sampling of water level fluctuation in the water tank are realized.

Description

Technical Field

[0001] The present invention relates to the field of water conservancy engineering experiments, and in particular to an experimental device and method for simulating the migration and transformation of nitrogen in groundwater in the riparian hyporheic zone. Background Art

[0002] The river hyporheic zone is the underground area near the riverbed and the riparian zone, where river water and groundwater mix, with high dynamics and randomness. Affected by hyporheic exchange, the biogeochemical reactions in the hyporheic zone are intense, which is an important place for the migration and transformation of nitrogen, and is crucial for regulating the ecological health of rivers.

[0003] The migration and transformation of nitrogen in the hyporheic zone are significantly affected by hydrological processes. Current research mainly focuses on hydrological processes such as flood processes, ebb and flow tides, or groundwater level fluctuations. Existing research has shown that the periodic water level fluctuations in tidal rivers can lead to frequent wet-dry cycles in the riparian zone, which is beneficial to denitrification and nitrogen removal in the riparian zone. In addition, frequent groundwater level fluctuations can cause oxygen in the vadose zone to enter the groundwater, affecting the redox environment of the riparian hyporheic zone, thereby reducing the denitrification and nitrogen removal capacity of the hyporheic zone to a certain extent. Regarding the research on nitrogen migration and transformation in the hyporheic zone, the current main research method is field monitoring. The composition of natural riparian zone sediments is complex, and environmental factors are variable. It is difficult to repeat the experiments, especially unable to analyze the contribution of individual factors to the nitrogen migration and transformation in the hyporheic zone.

[0004] Therefore, scientific researchers have developed some related experimental devices and methods around hyporheic zone hydraulic exchange and solute transport, which are mainly applicable to coarse-grained sediment environments dominated by medium and coarse sands. However, in the middle and lower reaches of rivers, most are fine-grained sediment environments dominated by silt or clayey silt. The effects of different soil types on hyporheic exchange and nitrogen migration and transformation vary greatly. Therefore, it is urgent to develop an experimental device and method for simulating nitrogen migration and transformation in the hyporheic zone suitable for fine-grained sediment environments. In addition, the water level fluctuation control, water level data recording, and water quality sample collection of existing indoor models are mainly completed manually, which is time-consuming and laborious. Therefore, how to improve the automation degree of water tank water level control, water level monitoring of pore water, and water quality sampling is also a problem to be solved in the research and development of experimental devices. Moreover, the design of experimental devices and methods must be guided by the concept of green development, saving resources and reducing energy consumption. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned existing technologies and provide an experimental device and method for simulating the migration and transformation of nitrogen in groundwater in the riparian hyporheic zone, which greatly shortens the sampling time of pore water in fine-grained soil and ensures the accuracy of experimental results; realizes the automatic control of water level fluctuations in the water tank, full-process monitoring, and rapid automatic sampling.

[0006] The present invention provides an experimental device for simulating the migration and transformation of nitrogen in groundwater in the riparian subsurface flow zone, including:

[0007] A detachable plexiglass tank, formed by enclosing with plexiglass tank component A, plexiglass tank component B, and plexiglass tank component C. The internal space of the detachable plexiglass tank is sequentially divided into a left-end water tank, a middle sand tank, and a right-end water tank from left to right by two groups of permeable baffle plates inserted on the left and right sides of the detachable plexiglass tank, which are respectively used to simulate the river channel, the riverbank, and the far-bank aquifer;

[0008] A water level control component, including: two constant head control modules respectively connected to the left-end water tank and the right-end water tank, and corresponding platform lifting modules;

[0009] A pore water level monitoring component, including: a plurality of piezometers arrayed and buried in the middle sand tank, a plurality of transparent scales pasted on the front and rear sides of the plexiglass water-sand combined tank, and a photography module placed behind the plexiglass tank;

[0010] A water quality sampling component, including: a pore water filtration module arrayed and buried inside the middle sand tank, a vacuum pumping module, and a water sample collection module connected to the two;

[0011] Further, the cross-section of the plexiglass tank component A is C-shaped, and the cross-sections of the plexiglass tank component B and the plexiglass tank component C are both "[-]" shaped. The three components are fixed and enclosed by bolts and water-stop rubber pads. Two pairs of card slots are respectively opened on the inner walls of the left and right sides of the plexiglass tank component A to fix two groups of permeable baffle plates inserted vertically. Filter materials are filled between the two permeable baffle plates in each group to prevent fine-grained soil in the sand tank from being washed into the water tanks on both sides; the bottom end of the plexiglass tank component A is provided with feet.

[0012] Further, the permeable sand-proof plate is made of a plexiglass plate with dense holes and is wrapped with a fine-mesh nylon gauze on the outside to prevent fine-grained test sand from being washed into the water tank by water.

[0013] Further, the bottom plates of the left-end water tank and the right-end water tank are both provided with drain ports with valves for flushing and draining the water tanks after the experiment. And communication interfaces with valves are respectively provided below the left side panel of the left-end water tank and the right side panel of the right-end water tank, which are connected to the corresponding constant head control modules. Circular holes are opened on the front panel and the rear panel of the middle sand tank, which are arranged in an array, and are respectively used for arranging the water quality sampling component and the pore water level monitoring component.

[0014] Further, the constant head control module includes a constant head control unit for the left-end water tank and a constant head control unit for the right-end water tank;

[0015] The constant head control unit of the left-end water tank includes: a first Mariotte bottle connected to the communication interface of the left-end water tank, a first water supply tank connected to the intake pipe of the first Mariotte bottle, a first peristaltic pump, and a first drainage tank connected to the valve-equipped outlet pipe of the first Mariotte bottle;

[0016] The constant head control unit of the right-end water tank includes: a second Mariotte bottle connected to the communication interface of the right-end water tank, a second water supply tank connected to the intake pipe of the second Mariotte bottle, a second peristaltic pump, and a second drainage tank connected to the valve-equipped outlet pipe of the second Mariotte bottle.

[0017] Furthermore, the first Mariotte bottle is connected to the communication interface of the left-end water tank through a first connecting hose, the first water supply tank is connected to the intake pipe of the first Mariotte bottle through a first inlet hose, the first peristaltic pump is arranged on the first inlet hose, and the first drainage tank is connected to the valve-equipped outlet pipe of the first Mariotte bottle through a first drainage hose;

[0018] The second Mariotte bottle is connected to the communication interface of the right-end water tank through a second connecting hose, the second water supply tank is connected to the intake pipe of the second Mariotte bottle through a second inlet hose, the second peristaltic pump is arranged on the second inlet hose, and the second drainage tank is connected to the valve-equipped outlet pipe of the second Mariotte bottle through a second drainage hose;

[0019] The top covers of the first Mariotte bottle and the second Mariotte bottle are both provided with valve-equipped outlet pipes and are respectively placed on the platforms of an electric lifting table and a manual lifting table.

[0020] Furthermore, the electric lifting table is provided with a lifting speed controller, and by setting the lifting process of the first Mariotte bottle, the change process of the water level in the left-side water tank is further controlled.

[0021] Furthermore, the pore water level monitoring assembly includes: glass piezometers arranged on the rear panel of the middle sand tank, transparent scales pasted on the front and rear panels of the detachable plexiglass tank, a camera capable of photographing the liquid levels in all glass piezometers and the rising and falling processes of the water levels in the two water tanks and its fixing bracket; the glass piezometers are inserted into the interior of the middle sand tank through circular holes and threaded joints provided on the middle sand tank. The threaded joint is internally provided with a hollow soft silicone hole plug to ensure the sealing performance at the socket of the glass piezometer, and the inserted end of the glass piezometer is wrapped with fine-mesh nylon gauze to prevent fine-grained soil from blocking the pipe orifice.

[0022] Furthermore, the water quality sampling assembly includes: straight-through joints respectively installed on both sides of each circular hole opened on the front panel of the middle sand tank, clay heads connected to the inner straight-through joints, two-way test tubes connected to the outer straight-through joints through a hose flow regulator, and a vacuum pump connected to the two-way test tubes through a suction pipe and a multi-way joint;

[0023] The clay heads are arranged in an array and buried inside the sand tank for filtering sediment in the pore water. The two-way test tubes appear in pairs with the clay heads and are marked with scales. Two tubes are inserted into their lids, which are respectively used for pore water collection and air extraction. The number of passes of the multi-way joint can be determined according to specific circumstances, which can realize simultaneous sampling at multiple positions with a relatively small number of vacuum pumps, improving the sampling efficiency.

[0024] The present invention also provides a method for simulating the migration and transformation of nitrogen in groundwater in the riparian hyporheic zone by using the above test device, including the following steps:

[0025] S1: Filling of test sand and layout of monitoring and sampling instruments:

[0026] S11: Lay a water-stop rubber pad at the joint of the acrylic tank component B and the acrylic tank component A, and then fix it with several bolts; Wrap the sampling fine-mesh nylon gauze around the four permeable baffle plates respectively, and then vertically insert them into the corresponding card slots in the acrylic tank component A one by one; Fill the filter material between two adjacent permeable baffle plates.

[0027] S12: Fill the low-permeability test sand into the middle sand tank by the method of sampling and tamping layer by layer; When the test sand is filled to the circular holes opened on the front and rear panels of the acrylic tank, install a threaded joint at the circular hole of the rear panel, wrap the water inlet end of the glass piezometer tube with fine-mesh nylon gauze, insert it into the threaded joint and push it into the middle sand tank; Install a straight-through joint on each side of the circular hole on the front panel, connect the clay head with the inner straight-through joint, and connect the two ends of the hose flow regulator with the outer straight-through joint and the two-way test tube respectively.

[0028] S13: Repeat step S12 until the filled test sand is close to the top of the acrylic tank component B, and then fix and enclose the acrylic tank component C with the acrylic tank component B and the acrylic tank component A by bolts and water-stop rubber pads to form a shape; Repeat step S12 until the height of the test sand in the middle sand tank is close to the top of the acrylic tank component A.

[0029] S14: Use a multi-way joint and an air extraction pipe to connect multiple adjacent two-way test tubes to a single vacuum pump. Based on this energy-saving measure, complete the installation of the air extraction module for all two-way test tubes in the test.

[0030] S15: Stick transparent scales on the front and rear panels of the acrylic tank, place a tripod directly behind the acrylic tank, and fix a camera above it. Adjust the positions of the tripod and the lens so as to be able to photograph the liquid levels of all piezometer tubes and the water levels of the two water tanks.

[0031] S2: Water level control of the water tank:

[0032] S21: Adjust the heights of the electric lifting platform and the manual lifting platform so that the bottom ends of the intake pipes of the first and second Mariotte bottles are respectively located at H 1 and H 2 height, and H 1 <H 2 , simulating the situation where the water level at the right end is higher than that at the left end, that is, the situation with base flow; the liquids in the first and second water supply tanks are deionized water;

[0033] S22: Close the valves of the outlet pipe of the second Mariotte bottle and the valve of the left end water tank connection interface, connect the second inlet hose to the intake pipe of the second Mariotte bottle, open the valve of the outlet pipe of the second Mariotte bottle and the valve of the right end water tank connection interface, and at the same time start the second peristaltic pump to supply water from the second water supply tank to the second Mariotte bottle until the water level in the right end water tank is flush with the bottom end of the intake pipe of the second Mariotte bottle, then close the second peristaltic pump and the valve of the outlet pipe of the second Mariotte bottle, and disconnect the second inlet hose from the intake pipe of the second Mariotte bottle. At this time, the water levels in both water tanks are maintained at the height of H 2 ;

[0034] S23: Open the valve of the left end water tank connection interface, and perform constant water level control for the left and right end water tanks according to the position of the water level in the water tank and the bottom end of the intake pipe of the Mariotte bottle connected to it; the specific operations are as follows: when the water level in the water tank is lower than the bottom end of the intake pipe of the Mariotte bottle connected to it through the hose, close the valve of the outlet pipe of the Mariotte bottle, connect the inlet hose to the intake pipe of the Mariotte bottle, open the valve of the outlet pipe of the Mariotte bottle, and at the same time start the peristaltic pump to supply water from the water supply tank to the Mariotte bottle until the water level in the water tank is flush with the bottom end of the intake pipe of the Mariotte bottle connected to it through the hose, then close the peristaltic pump and the valve of the outlet pipe of the Mariotte bottle, and disconnect the inlet hose from the intake pipe of the Mariotte bottle; when the water level in the water tank is higher than the bottom end of the intake pipe of the Mariotte bottle connected to it through the hose, the liquid level in the Mariotte bottle rises and the water level in the water tank drops until it is flush with the bottom end of the intake pipe of the Mariotte bottle. If the liquid level in the Mariotte bottle is close to the bottle cap during the rising process, the valve of the outlet pipe of the Mariotte bottle needs to be opened to drain water to the drainage tank through the drainage hose until the liquid level in the Mariotte bottle is not lower than the top end of the outlet pipe, then close the outlet pipe valve;

[0035] S24: Through the above series of operations of constant water level control, the water levels in the left and right end water tanks are respectively stabilized at H 1 and H 2 ; According to the characteristics of the tidal process or flood process and the size of the plexiglass tank, determine the process line simulated by the water level fluctuation in the left end water tank during the test, and its initial water level is H 1 ; Collect water level data from the simulated process line at time intervals of Δt 1 , so as to obtain the water level change rate in each period, that is, the lifting control rate of the electric lifting platform in each period;

[0036] S25: Turn on the lifting speed controller, set the lifting rates for each period, and repeat for several cycles; replace the solution in water supply tank 1 with ammonium chloride solution of a given concentration; turn on the electric lifting platform and lift and lower it at the set rates; during the lifting and lowering of the electric lifting platform, conduct constant water level control on the water levels in the water tanks at both ends to ensure that the water level in the left water tank fluctuates according to the set simulation curve and the water level in the right water tank remains at height H; when the water level in the left water tank completes the set fluctuation and returns to the initial position, the test ends. 2 When the water level in the left water tank completes the set fluctuation and returns to the initial position, the test ends.

[0037] S3: Pore water level monitoring and water quality sampling:

[0038] S31: During the test, use a camera to record the liquid level positions in each piezometer tube on the rear panel of the plexiglass tank, the water levels in the two water tanks, and their corresponding times throughout the process.

[0039] S32: During the fluctuation of the water level in the left water tank, starting from the initial moment, open the water supply switch of the hose flow rate regulator and the vacuum pump at time intervals of Δt to collect a certain volume of pore water samples from the sand tank. While collecting the pore water samples, also collect water samples of the same volume from the left and right water tanks. After each collection of water samples, turn off the vacuum pump, set the adjustment switch of the hose flow rate regulator to the water stop state, and replace with brand-new two-way test tubes; put the pore water and the water samples from the two water tanks into a low-temperature box; after the entire test ends, send all the water samples to the laboratory for determination of the concentrations of the three nitrogens. 2 After each collection of water samples, turn off the vacuum pump, set the adjustment switch of the hose flow rate regulator to the water stop state, and replace with brand-new two-way test tubes; put the pore water and the water samples from the two water tanks into a low-temperature box; after the entire test ends, send all the water samples to the laboratory for determination of the concentrations of the three nitrogens.

[0040] S4: Data processing and analysis:

[0041] S41: Starting from the initial moment of the test, collect the positions of the liquid levels in all glass piezometer tubes and the water levels in the two water tanks in the captured video at time intervals of Δt; according to the burial depths of each glass piezometer tube and the information collected in the video, draw the two-dimensional pore water flow field map of the middle sand tank at different times, so as to obtain the variation rules of the subsurface flow hydraulic exchange rate and flux in the middle sand tank during the fluctuation of the water level in the left sand tank. 2 According to the burial depths of each glass piezometer tube and the information collected in the video, draw the two-dimensional pore water flow field map of the middle sand tank at different times, so as to obtain the variation rules of the subsurface flow hydraulic exchange rate and flux in the middle sand tank during the fluctuation of the water level in the left sand tank.

[0042] S42: According to the data of the ammonia nitrogen, nitrate nitrogen, and nitrite nitrogen concentrations of the collected water samples, draw the distribution map of the three nitrogen concentrations in the pore water of the middle sand tank at time intervals of Δt and the penetration curves of the ammonia nitrogen concentrations at different positions, and reveal the influence rules of the river water level fluctuation on the nitrogen migration range and transformation rate in the riparian subsurface flow zone. 2 According to the data of the ammonia nitrogen, nitrate nitrogen, and nitrite nitrogen concentrations of the collected water samples, draw the distribution map of the three nitrogen concentrations in the pore water of the middle sand tank at time intervals of Δt and the penetration curves of the ammonia nitrogen concentrations at different positions, and reveal the influence rules of the river water level fluctuation on the nitrogen migration range and transformation rate in the riparian subsurface flow zone.

[0043] Compared with the prior art, the present invention has the following advantages:

[0044] (1) The present invention uses a clay head to filter pore water, greatly reducing the content of fine particles such as silt or silt loam in the water quality sampling bottle. Moreover, combined with sampling using a vacuum pump, the sampling time of pore water in fine-grained soil is greatly shortened, ensuring the accuracy of test results. In addition, well-graded filter materials are filled between the water tank and the sand tank, reducing the loss of fine-grained soil in the sand tank to the two side water tanks.

[0045] (2) The present invention uses a lifting speed controller to control the lifting of the Mariotte bottle according to the set water level fluctuation line, thereby realizing the automatic control of the water level fluctuation in the water tank. At the same time, a camera is used to monitor the whole process of the water level change in the water tank and the pore water level change in the sand tank. In addition, negative pressure is generated by a vacuum pump, and multi-way joints are combined to achieve multi-point synchronous and rapid automatic sampling.

[0046] (3) The present invention uses detachable components to assemble the plexiglass tank. In particular, the rear panel of the plexiglass can be disassembled into two pieces, which is convenient for the layered compaction of the test sand and the installation of test devices such as the clay head and joints at the bottom of the sand tank. It is also beneficial for the cleaning of the sand after the test and the flushing of the plexiglass tank.

[0047] (4) The pore water sampling system of the present invention has good airtightness, reducing the contact time between the pore water sample and the outside atmosphere and ensuring the accuracy of the water quality analysis results.

[0048] (5) The present invention uses a two-dimensional water tank - sand tank combination device to synchronously study the hydraulic exchange and nitrogen migration and transformation laws in the hyporheic zone, providing a useful tool for the study of the driving mechanism of nitrogen cycling in groundwater in the hyporheic zone. Description of the Drawings

[0049] Figure 1 It is a schematic diagram of the overall structure of the test device for simulating nitrogen migration and transformation in groundwater in the riparian hyporheic zone;

[0050] Figure 2 It is a front view of the middle structure of the test device for simulating nitrogen migration and transformation in groundwater in the riparian hyporheic zone;

[0051] Figure 3 It is a rear view of the middle structure of the test device for simulating nitrogen migration and transformation in groundwater in the riparian hyporheic zone;

[0052] Figure 4 It is a top view of the middle structure of the test device for simulating nitrogen migration and transformation in groundwater in the riparian hyporheic zone;

[0053] Figure 5 It is a cross-sectional view of the middle structure of the test device for simulating nitrogen migration and transformation in groundwater in the riparian hyporheic zone.

[0054] Figure numerals: 1. Organic glass groove component A, 2. Organic glass groove component B, 3. Organic glass groove component C, 4. Bolt, 5. Water-stop rubber pad, 6. Card slot, 7. Permeable sand baffle, 8. Filter material, 9. Drain port, 10. Connecting interface, 11. Circular hole, 12. Support foot, 13. Connecting hose 1, 14. Connecting hose 2, 15. Martens flask 1, 16. Martens flask 2, 17. Air inlet pipe, 18. Air outlet pipe, 19. Water outlet pipe, 20. Water inlet hose 1, 21. Water inlet hose 2, 22. Water supply tank 1, 23. Water supply tank 2, 24. Peristaltic pump 1, 25. Peristaltic pump 2, 26. Drain hose 1, 27. Drain hose 2, 28. Drain box 1, 29. Drain box 2, 30. Electric lifting platform, 31. Manual lifting platform, 32. Lifting speed controller, 33. Glass pressure measuring tube, 34. Transparent scale, 35. Camera, 36. Fixed bracket, 37. Threaded joint, 38. Straight joint, 39. Clay head, 40. Hose flow rate regulator, 41. Two-way test tube, 42. Vacuum pipe, 43. Multi-way joint, 44. Vacuum pump, 45. Test sand. DETAILED DESCRIPTION

[0055] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. Component models, material names, connection structures, control methods, algorithms and other features not clearly described in this technical solution are all considered to be common technical features disclosed in the prior art.

[0056] Example 1

[0057] This embodiment provides a test device for simulating the migration and transformation of nitrogen in groundwater in the river bank undercurrent zone. Figures 1-5 As shown, including:

[0058] The removable organic glass tank is formed by the organic glass tank component A1, the organic glass tank component B2 and the organic glass tank component C3. The internal space of the removable organic glass tank is divided into a left end water tank, a middle sand tank and a right end water tank from left to right by two sets of permeable sand retaining plates inserted on the left and right sides of the removable organic glass tank, which are used to simulate the river channel, the river bank and the far bank aquifer respectively;

[0059] The water level control assembly includes: two constant water head control modules connected to the left end water tank and the right end water tank respectively and a platform lifting module corresponding thereto;

[0060] The pore water level monitoring assembly includes: a plurality of pressure measuring tubes buried in an array in the middle sand tank, a plurality of transparent scales posted on the front and rear sides of the organic glass water-sand combination tank, and a photography module placed behind the organic glass tank;

[0061] The water quality sampling component includes a pore water filtration module arrayed and buried inside the middle sand tank, a vacuum pumping module, and a water sample collection module connected to both of them.

[0062] In the specific implementation manner, the cross-section of the plexiglass tank component A1 is C-shaped, and the cross-sections of the plexiglass tank components B2 and C3 are both "[-shaped. The three components are fixed and enclosed by bolts 4 and water-stop rubber gaskets 5. On the inner walls of the left and right side parts of the plexiglass tank component A1, two pairs of card slots 6 are respectively opened to fix two groups of water-permeable sand retaining plates 7 inserted vertically. Filter materials 8 are filled between the two water-permeable sand retaining plates 7 in each group to prevent fine-grained soil in the sand tank from being washed into the water tanks on both sides; the bottom end of the plexiglass tank component A1 is provided with supporting feet 12.

[0063] The water-permeable sand-proof plate 7 is made of a plexiglass plate with dense holes and is wrapped with a nylon gauze with fine mesh on the outside to prevent fine-grained test sand from being washed into the water tank by water.

[0064] The bottom plates of the left water tank and the right water tank are both provided with drain ports 9 with valves, which are used for flushing and draining the water tanks after the test. And communication interfaces 10 with valves are respectively provided below the left side panel of the left water tank and the right side panel of the right water tank, and are connected to the corresponding constant head control modules. Circular holes 11 arranged in an array of 4 rows × 6 columns are opened on the front panel and the rear panel of the middle sand tank, and their diameter is 2 cm. The positions of the circular holes 11 on the front and rear panels are the same. Horizontally, the circular holes 11 are gradually sparser from left to right, and the distances from the left boundary of the middle sand tank to the water-permeable sand retaining plates 7 are 10 cm, 25 cm, 45 cm, 75 cm, 115 cm, and 165 cm respectively; vertically, the distances from the circular holes 11 to the inner wall of the plexiglass tank bottom plate from bottom to top are 10 cm, 25 cm, 45 cm, and 65 cm respectively.

[0065] The plexiglass tank is processed from a plexiglass plate with a thickness of 2 cm. The total length of the detachable plexiglass tank is 248 cm and the height is 100 cm. Four pairs of card slots 6 are provided on the inner walls of its front and rear sides. Four water-permeable sand retaining plates 7 are vertically placed by inserting into each pair of card slots 6. The water-permeable sand retaining plates 7 are plexiglass plates with dense holes and a thickness of 1 cm, and are wrapped with 80-mesh nylon gauze on the outside. Filter materials 8 are arranged between two adjacent water-permeable sand retaining plates 7 to prevent the scouring of the test sand by water flow. The length of the single-sided filter material is 7 cm and the width is 15 cm. The filter materials 8 on the left and right sides and the two water-permeable sand retaining plates 7 connected thereto divide the internal space of the detachable plexiglass tank into a left water tank, a middle sand tank, and a right water tank. Among them, the internal spaces of the left and right water tanks are both 15 cm in length and 15 cm in width, and the middle sand tank is 200 cm in length and 15 cm in width. Multiple supporting feet are provided at the bottom end of the plexiglass tank.

[0066] In the specific implementation manner, the constant head control module includes a constant head control unit for the left-end water tank and a constant head control unit for the right-end water tank;

[0067] The constant head control unit for the left-end water tank includes: a first Mariotte bottle 15 connected to the connection interface 10 of the left-end water tank, a first water supply tank 22 connected to the air inlet pipe 17 of the first Mariotte bottle 15, a first peristaltic pump 24, and a first drainage tank 28 connected to the valve-equipped outlet pipe 19 of the first Mariotte bottle 15;

[0068] The constant head control unit for the right-end water tank includes: a second Mariotte bottle 16 connected to the connection interface 10 of the right-end water tank, a second water supply tank 23 connected to the air inlet pipe 17 of the second Mariotte bottle 16, a second peristaltic pump 25, and a second drainage tank 29 connected to the valve-equipped outlet pipe 19 of the second Mariotte bottle 16.

[0069] In the specific implementation manner, the first Mariotte bottle 15 is connected to the connection interface 10 of the left-end water tank through a first connecting hose 13, the first water supply tank 22 is connected to the air inlet pipe 17 of the first Mariotte bottle 15 through a first water inlet hose 20, the first peristaltic pump 24 is arranged on the first water inlet hose 20, and the first drainage tank 28 is connected to the valve-equipped outlet pipe 19 of the first Mariotte bottle 15 through a first drainage hose 26;

[0070] The second Mariotte bottle 16 is connected to the connection interface 10 of the right-end water tank through a second connecting hose 14, the second water supply tank 23 is connected to the air inlet pipe 17 of the second Mariotte bottle 16 through a second water inlet hose 21, the second peristaltic pump 25 is arranged on the second water inlet hose 21, and the second drainage tank 29 is connected to the valve-equipped outlet pipe 19 of the second Mariotte bottle 16 through a second drainage hose 27;

[0071] The top covers of the first Mariotte bottle 15 and the second Mariotte bottle 16 are both provided with valve-equipped air outlet pipes 18, and are respectively placed on the platforms of an electric lifting table 30 and a manual lifting table 31.

[0072] In the specific implementation manner, the electric lifting table 30 is provided with a lifting speed controller 32, and by setting the lifting process of the first Mariotte bottle 15, the change process of the water level in the left-side water tank is controlled accordingly.

[0073] In the specific embodiment, the pore water level monitoring assembly includes: a glass piezometer tube 33 arranged on the rear panel of the middle sand tank, a transparent scale 34 attached to the front and rear panels of the detachable plexiglass tank, a camera 35 capable of photographing the liquid levels in all the glass piezometer tubes 33 and the water level fluctuations in the two water tanks, and its fixed bracket 36; the glass piezometer tube 33 is inserted into the interior of the middle sand tank through a circular hole 11 and a threaded joint 37 provided on the middle sand tank. The threaded joint 37 is internally provided with a hollow soft silica gel hole plug to ensure the sealing performance at the socket of the glass piezometer tube 33, and the insertion end of the glass piezometer tube 33 is wrapped with a fine-mesh nylon gauze to prevent fine-grained soil from clogging the tube orifice.

[0074] In the specific embodiment, the water quality sampling assembly includes: straight-through joints 38 respectively installed on both sides of each circular hole 11 opened on the front panel of the middle sand tank, a clay head 39 connected to the inner straight-through joint 38, a two-way test tube 41 connected to the outer straight-through joint 38 through a hose flow regulator 40, and a vacuum pump 44 connected to the two-way test tube 41 through a suction pipe 42 and a multi-way joint 43;

[0075] The clay heads 38 are arranged in an array and buried inside the sand tank for filtering sediment in the pore water. The two-way test tubes 41 appear in pairs with the clay heads 38 and are marked with scales on them. Two tubes are inserted into their lids, which are respectively used for pore water collection and air extraction.

[0076] This embodiment also provides a method for simulating the migration and transformation of nitrogen in groundwater in the riparian hyporheic zone by using the above test device, including the following steps:

[0077] S1: Filling of the test sand 45 and arrangement of monitoring and sampling instruments:

[0078] S11: Lay a water-stop rubber pad 5 at the joint between the plexiglass tank component B2 and the plexiglass tank component A1, and then fix it with several bolts 4; wrap the four permeable baffle sand plates 7 with sampling fine-mesh nylon gauze respectively, and then vertically insert them into the corresponding card slots 6 in the plexiglass tank component A1 one by one; fill the filter material 8 between two adjacent permeable baffle sand plates 7;

[0079] S12: Fill the low-permeability test sand 45 (such as silt) into the middle sand tank by the method of sampling, layering and ramming; when the test sand 45 is filled up to the circular holes 11 opened on the front and rear panels of the plexiglass tank, install a threaded joint 37 at the circular hole on the rear panel, wrap the water inlet end of the glass piezometer tube 33 with a fine-mesh nylon gauze, insert it into the threaded joint 37 and push it into the interior of the middle sand tank; install a straight-through joint 38 on each side of the circular hole on the front panel, connect the ceramic tip 39 with the inner straight-through joint 38, and connect the two ends of the hose flow regulator 40 with the outer straight-through joint 38 and the two-way test tube 41 respectively; the length range of the glass piezometer tube 33 is 25 - 80 cm, the inner diameter is 8 mm, and the outer diameter is 10 mm; the capacity of the two-way test tube 41 is 50 ml.

[0080] S13: Repeat step S12 until the filled test sand 45 is near the top of the plexiglass tank component B2, and then fix and enclose the plexiglass tank component C3, the plexiglass tank component B2 and the plexiglass tank component A1 into a shape by bolts 4 and the water-stop rubber gasket 5; repeat step S12 until the height of the test sand 45 in the middle sand tank is near the top of the plexiglass tank component A1;

[0081] S14: Use a multi-way joint 43 and an air extraction tube 42 to connect multiple adjacent two-way test tubes 41 with a vacuum pump 44, and complete the installation of the air extraction module for all two-way test tubes 41 in the experiment based on this energy-saving measure;

[0082] S15: Paste a transparent scale 34 on the front and rear panels of the plexiglass tank, place a tripod directly behind the plexiglass tank, and fix a camera 35 above it. Adjust the positions of the tripod and the lens so as to be able to photograph the liquid levels of all piezometer tubes and the water levels of the two water tanks;

[0083] S2: Water level control of the water tank:

[0084] S21: Adjust the heights of the electric lifting platform 30 and the manual lifting platform 31 so that the bottom ends of the inlet pipes of the first Mariotte bottle 15 and the second Mariotte bottle 16 are respectively 30 cm and 35 cm higher than the inner bottom surface of the plexiglass tank, simulating the situation where the water level on the right end is higher than that on the left end, that is, the situation of base flow; the liquids in the first water supply tank 22 and the second water supply tank 23 are deionized water; the ranges of the first peristaltic pump 24 and the second peristaltic pump 25 are the same, and the flow control range is between 0.1 ml / min and 1.6 L / min. The maximum lifting height of the electric lifting platform 30 and the manual lifting platform 31 is 50 cm.

[0085] S22: Close the valves of the water outlet pipe 19 of the second Mariotte bottle 16 and the valve of the left-end water tank connection interface 10. Connect the second water inlet hose 21 to the air inlet pipe 17 of the second Mariotte bottle 16. Open the valve of the air outlet pipe 18 of the second Mariotte bottle 16 and the valve of the right-end water tank connection interface 10. At the same time, start the second peristaltic pump 25 to supply water from the second water supply tank 23 to the second Mariotte bottle 16 until the water level in the right-end water tank is flush with the bottom end of the air inlet pipe 17 of the second Mariotte bottle 16. Then, close the second peristaltic pump 25 and the valve of the air outlet pipe 18 of the second Mariotte bottle 16, and disconnect the second water inlet hose 21 from the air inlet pipe 17 of the second Mariotte bottle 16. At this time, the water levels in both water tanks are maintained at a height of 35 cm.

[0086] S23: Open the valve of the left-end water tank connection interface 10. According to the position of the water level in the water tank and the bottom end of the air inlet pipe of the Mariotte bottle connected thereto, perform constant water level control for the left and right water tanks. The specific operations are as follows: When the water level in the water tank is lower than the bottom end of the air inlet pipe 17 of the Mariotte bottle connected to it through the hose, close the valve of the water outlet pipe 19 of the Mariotte bottle, connect the water inlet hose to the air inlet pipe 17 of the Mariotte bottle, open the valve of the air outlet pipe 18 of the Mariotte bottle, and at the same time start the peristaltic pump to supply water from the water supply tank to the Mariotte bottle until the water level in the water tank is flush with the bottom end of the air inlet pipe 17 of the Mariotte bottle connected to it through the hose. Then, close the peristaltic pump and the valve of the air outlet pipe 18 of the Mariotte bottle, and disconnect the water inlet hose from the air inlet pipe 17 of the Mariotte bottle; When the water level in the water tank is higher than the bottom end of the air inlet pipe 17 of the Mariotte bottle connected to it through the hose, the liquid level in the Mariotte bottle rises and the water level in the water tank drops until it is flush with the bottom end of the air inlet pipe 17 of the Mariotte bottle. If the liquid level in the Mariotte bottle is close to the bottle cap during the rising process, open the valve of the water outlet pipe 19 of the Mariotte bottle and drain water to the drainage tank through the drainage hose until the liquid level in the Mariotte bottle is not lower than the top end of the water outlet pipe 19, and then close the water outlet valve.

[0087] S24: Through the above series of operations of constant water level control, the water levels in the left and right water tanks are stabilized at 30 cm and 35 cm respectively. According to the characteristics of the tidal process or flood process and the size of the plexiglass tank, determine the process line of the water level fluctuation simulation in the left water tank during the experiment. Combining with the size of the plexiglass tank, the amplitude of the sine curve is 15 cm, the period is 12 h, and the initial water level is 30 cm, that is, the water level changes from 15 cm to 45 cm; The water level in the right water tank is fixed at 35 cm to simulate the far-shore groundwater not affected by tides. Collect water level data from the sine curve at 30-minute time intervals to obtain the water level change rate at each time period, that is, the lifting control rate of the electric lifting platform 30 at each time period.

[0088] S25: Turn on the lifting speed controller 32, set the lifting rates for each period, and repeat for 3 cycles; replace the solution in water supply tank 1 - 22 with ammonium chloride solution at a concentration of 100 mg / L; turn on the electric lifting platform and lift and lower it at the set rates; during the lifting and lowering of the electric lifting platform, perform constant water level control on the water levels in the water tanks at both ends to ensure that the water level in the left - hand water tank fluctuates sinusoidally and the water level in the right - hand water tank remains unchanged. When the water level in the left - hand water tank has completed 3 cycles of sinusoidal fluctuations continuously, the test ends.

[0089] S3: Pore water level monitoring and water quality sampling:

[0090] S31: During the test, use camera 35 to record the liquid level positions in each piezometer tube on the rear panel of the plexiglass tank, the water levels in the two water tanks, and their corresponding times throughout the process;

[0091] S32: During the fluctuation of the water level in the left - hand water tank, starting from the initial moment, open the water - passing switch of the hose flow regulator 40 and the vacuum pump 44 at 60 - minute time intervals to collect 20 - ml pore water water samples. While collecting the pore water water samples, collect water samples of the same volume from the left - hand and right - hand water tanks. After each collection of water samples, turn off the vacuum pump, set the regulating switch of the hose flow regulator 40 to the water - stopping state, and replace with a brand - new two - way test tube. Place the pore water and the water samples from the two water tanks in a low - temperature box. After the entire test is completed, send all the water samples to the laboratory for determination of the concentrations of the three nitrogen species;

[0092] S4: Data processing and analysis:

[0093] S41: Starting from the initial moment of the test, collect the liquid level positions in all glass piezometer tubes 33 and the water levels in the two water tanks in the video taken at 60 - minute time intervals; based on the burial depths of each glass piezometer tube 33 and the information collected in the video, draw the two - dimensional pore water flow field map of the middle sand tank at different times, so as to obtain the variation laws of the subsurface flow hydraulic exchange rate and flux in the middle sand tank during the water level fluctuation in the left - hand sand tank;

[0094] S42: According to the data of the ammonia nitrogen, nitrate nitrogen, and nitrite nitrogen concentrations of the collected water samples, draw the distribution map of the three nitrogen species concentrations in the pore water of the middle sand tank at 60 - minute time intervals and the breakthrough curves of the ammonia nitrogen concentrations at different positions, and reveal the influence laws of river water level fluctuations on the nitrogen migration range and transformation rate in the riparian subsurface flow zone.

[0095] The components not elaborated in detail in this embodiment are all existing components that can be purchased from public channels.

[0096] The above description of the embodiments is provided to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A test device for simulating nitrogen migration and transformation in groundwater in a riverbank hyporheic zone, characterized in that: include: The removable organic glass tank is formed by the organic glass tank component A (1), the organic glass tank component B (2) and the organic glass tank component C (3). The internal space of the removable organic glass tank is divided into a left end water tank, a middle sand tank and a right end water tank from left to right by two sets of water-permeable sand retaining plates inserted on the left and right sides of the removable organic glass tank, respectively for simulating a river channel, a river bank and a far-shore aquifer; The water level control assembly includes: two constant water head control modules connected to the left end water tank and the right end water tank respectively and a platform lifting module corresponding thereto; The pore water level monitoring assembly includes: a plurality of pressure measuring tubes buried in an array in the middle sand tank, a plurality of transparent scales posted on the front and rear sides of the organic glass water-sand combination tank, and a photography module placed behind the organic glass tank; The water quality sampling component includes: a pore water filtering module buried in an array inside a middle sand tank, a vacuum pumping module, and a water sample collecting module connected to the two.

2. The experimental device for simulating nitrogen migration and transformation in groundwater in a riverbank hyporheic zone according to claim 1, characterized in that: The cross section of the organic glass trough component A (1) is C-shaped, and the cross sections of the organic glass trough component B (2) and the organic glass trough component C (3) are both "["-shaped. The three components are fixed and enclosed by bolts (4) and waterproof rubber pads (5). Two pairs of slots (6) are respectively provided on the inner walls of the left and right sides of the organic glass trough component A (1), and two groups of water-permeable sand retaining plates (7) are respectively fixed and vertically inserted. Filter material (8) is filled between the two water-permeable sand retaining plates (7) of each group to prevent fine particles of soil in the sand trough from being washed into the water troughs on both sides. The bottom end of the organic glass trough component A (1) is provided with a support foot (12).

3. The experimental device for simulating nitrogen migration and transformation in groundwater in the riverbank hyporheic zone according to claim 2, characterized in that: The water-permeable sand-proof plate (7) is a dense-pore organic glass plate, and the outside is wrapped with fine-mesh nylon gauze to prevent fine-grained test sand from being washed into the water tank by water.

4. The experimental device for simulating nitrogen migration and transformation in groundwater in a riverbank hyporheic zone according to claim 1, characterized in that: The bottom plates of the left and right water tanks are both provided with drain ports (9) with valves for flushing and draining the water tanks after the test, and the lower parts of the left side panel of the left water tank and the right side panel of the right water tank are both provided with connecting interfaces (10) with valves for connecting with the corresponding constant water head control modules, and the front and rear panels of the middle sand tank are both provided with circular holes (11) arranged in an array, for respectively arranging the water quality sampling components and the pore water level monitoring components.

5. The experimental device for simulating nitrogen migration and transformation in groundwater in riverbank hyporheic zone according to claim 1, characterized in that: The constant water head control module includes a constant water head control unit of the left end water tank and a constant water head control unit of the right end water tank; The constant water head control unit of the left end water tank comprises: a Malvern flask (15) connected to the communication interface (10) of the left end water tank, a water supply tank (22) connected to the air inlet pipe (17) of the Malvern flask (15), a peristaltic pump (24), and a water discharge tank (28) connected to the water outlet pipe (19) of the Malvern flask (15); The constant water head control unit of the right end water tank comprises: a second Malchnitz flask (16) connected to the communication interface (10) of the right end water tank, a second water supply tank (23) connected to the air inlet pipe (17) of the second Malchnitz flask (16), a second peristaltic pump (25), and a second water discharge tank (29) connected to the water outlet pipe (19) of the second Malchnitz flask (16).

6. The experimental device for simulating nitrogen migration and transformation in groundwater in the riverbank hyporheic zone according to claim 5, characterized in that: The Malchnitz flask (15) is connected to the communication interface (10) of the left water tank through a communication hose (13); the water supply tank (22) is connected to the air inlet pipe (17) of the Malchnitz flask (15) through a water inlet hose (20); a peristaltic pump (24) is arranged on the water inlet hose (20); and a drainage tank (28) is connected to the water outlet pipe (19) with a valve of the Malchnitz flask (15) through a drainage hose (26); The second Malchnitz flask (16) is connected to the communication interface (10) of the right end water tank through the second communication hose (14), the second water supply tank (23) is connected to the air inlet pipe (17) of the second Malchnitz flask (16) through the second water inlet hose (21), the second peristaltic pump (25) is arranged on the second water inlet hose (21), and the second drainage tank (29) is connected to the valved water outlet pipe (19) of the second Malchnitz flask (16) through the second drainage hose (27); The top covers of the first Malchnitz flask (15) and the second Malchnitz flask (16) are both provided with an air outlet pipe (18), and are placed on the platforms of an electric lifting platform (30) and a manual lifting platform (31) respectively.

7. The test device for simulating nitrogen migration and transformation in groundwater in a riverbank hyporheic zone according to claim 6, characterized in that: The electric lifting platform (30) is provided with a lifting speed controller (32) which controls the change process of the water level in the left water tank by setting the lifting process of the Martens flask (15).

8. The experimental device for simulating nitrogen migration and transformation in groundwater in a riverbank hyporheic zone according to claim 1, characterized in that: The pore water level monitoring assembly comprises: a glass pressure measuring tube (33) arranged on the rear panel of the middle sand tank, a transparent scale (34) attached to the front panel and the rear panel of the detachable organic glass tank, a camera (35) and a fixing bracket (36) thereof; the glass pressure measuring tube (33) is inserted into the middle sand tank through a circular hole (11) and a threaded joint (37) provided on the middle sand tank.

9. The experimental device for simulating nitrogen migration and transformation in groundwater in riverbank hyporheic zone according to claim 1, characterized in that: The water quality sampling assembly comprises: a straight-through joint (38), a clay head (39) connected to the inner straight-through joint (38), a two-way test tube (41) connected to the outer straight-through joint (38) via a hose flow rate regulator (40), and a vacuum suction pump (44) connected to the two-way test tube (41) via an air suction pipe (42) and a multi-way joint (43); The clay heads (38) are embedded in the sand tank in an array and are used to filter the sediment in the pore water. The two-way test tube (41) is paired with the clay heads (38) and is marked with scales. The cover is inserted with two tubes, which are used for pore water collection and air extraction respectively.

10. A method for simulating nitrogen migration and transformation in groundwater in a riverbank hyporheic zone using the test device according to any one of claims 1 to 9, characterized in that: The following steps are involved: S1: Test sand (45) filling and layout of monitoring and sampling instruments: S11: A waterproof rubber pad (5) is placed at the joint between the organic glass tank component B (2) and the organic glass tank component A (1), and then fixed with a plurality of bolts (4); four water-permeable sand retaining plates (7) are respectively wrapped with sampling fine mesh nylon gauze, and then vertically inserted into the corresponding slots (6) in the organic glass tank component A (1); filter material (8) is filled between two adjacent water-permeable sand retaining plates (7); S12: Fill the low-permeability test sand (45) into the middle sand tank by the method of sampling, layering and ramming; when the test sand (45) is filled to the circular holes (11) opened on the front and rear panels of the plexiglass tank, install a threaded joint (37) at the circular hole on the rear panel, wrap the water inlet end of the glass piezometer tube (33) with a fine mesh nylon gauze, insert it into the threaded joint (37) and push it into the interior of the middle sand tank; install a straight-through joint (38) on each side of the circular hole on the front panel, connect the ceramic head (39) with the inner straight-through joint (38), and connect the two ends of the hose flow regulator (40) with the outer straight-through joint (38) and the two-way test tube (41) respectively; S13: Repeat step S12 until the filled test sand (45) is near the top of the plexiglass tank component B (2), and then fix and enclose the plexiglass tank component C (3) with the plexiglass tank component B (2) and the plexiglass tank component A (1) into a shape by bolts (4) and water-stop rubber pads (5); repeat step S12 until the height of the test sand (45) in the middle sand tank is near the top of the plexiglass tank component A (1); S14: Use a multi-way joint (43) and an air extraction pipe (42) to connect multiple adjacent two-way test tubes (41) with a vacuum pump (44) to complete the installation of the air extraction module for all two-way test tubes (41) in the test; S15: Paste a transparent scale (34) on the front and rear panels of the plexiglass tank, place a tripod directly behind the plexiglass tank, and fix a camera (35) above it. Adjust the positions of the tripod and the lens so as to be able to photograph the liquid levels of all piezometer tubes and the water levels of the two water tanks; S2: Water level control of the water tank: S21: Adjust the heights of the electric lifting platform (30) and the manual lifting platform (31) so that the bottom ends of the inlet pipes of the first Mariotte bottle (15) and the second Mariotte bottle (16) are located at the heights of H1 and H2 respectively, and H1 < H2, simulating the situation where the water level at the right end is higher than that at the left end, that is, the situation of base flow; the liquids in the first water supply tank (22) and the second water supply tank (23) are deionized water; S22: Close the valves of the outlet pipe (19) of the second Mariotte bottle (16) and the valve of the left-end water tank connection interface (10), connect the second inlet hose (21) with the inlet pipe (17) of the second Mariotte bottle (16), open the valves of the outlet pipe (18) of the second Mariotte bottle (16) and the right-end water tank connection interface (10), and start the second peristaltic pump (25) at the same time to supply water from the second water supply tank (23) to the second Mariotte bottle (16) until the water level of the right-end water tank is flush with the bottom end of the inlet pipe (17) of the second Mariotte bottle (16), then close the second peristaltic pump (25) and the valve of the outlet pipe (18) of the second Mariotte bottle (16), and disconnect the second inlet hose (21) from the inlet pipe (17) of the second Mariotte bottle (16). At this time, the water levels of the two water tanks are maintained at the height of H2; S23: Open the valve of the left-end water tank connection interface (10), and conduct constant water level control of the left and right water tanks according to the positions of the water levels in the water tanks and the bottom ends of the inlet pipes of the Mariotte bottles connected thereto; S24: through the above-mentioned series of operations of constant water level control, the water levels of the left and right water tanks are stabilized at H1 and H2 respectively; according to the characteristics of the tidal process or flood process and the size of the organic glass tank, the process line of the water level fluctuation simulation of the left water tank in the test is determined, and its initial water level is H1; water level data is collected from the simulated process line at a time interval of Δt1, so as to obtain the water level change rate in each time period, that is, the lifting control rate of the electric lifting platform (30) in each time period; S25: Turn on the lifting speed controller (32), set the lifting speed in each time period, and repeat several cycles; replace the solution in the water supply tank (22) with an ammonium chloride solution of a given concentration; turn on the electric lifting platform and lift it at the set speed; during the lifting process of the electric lifting platform, the water levels of the water tanks at both ends are controlled to be constant to ensure that the water level of the left water tank fluctuates according to the set simulation curve and the water level of the right water tank is maintained at the height H2; when the water level of the left water tank completes the set fluctuation and returns to the initial position, the test ends; S3: Pore water level monitoring and water quality sampling: S31: During the test, a camera (35) is used to record the liquid level positions in the pressure measuring tubes on the rear panel of the organic glass tank, the water levels in the two water tanks and the corresponding time; S32: During the fluctuation of the water level in the left water tank, starting from the initial moment, the water switch of the hose flow rate regulator (40) and the vacuum pump (44) are turned on at a time interval of Δt2 to collect a certain volume of sand tank pore water samples. While collecting the pore water samples, the same volume of water samples are also collected from the left and right water tanks. After each water sample is collected, the vacuum pump (44) is turned off, and the regulating switch of the hose flow rate regulator (40) is set to the water-stopping state, and a new two-way test tube (41) is replaced; the pore water and water samples from the two water tanks are placed in a low-temperature box; after all the tests are completed, all water samples are sent to the laboratory to measure the nitrogen concentration; S4: Data processing and analysis: S41: starting from the initial moment of the test, the positions of the liquid levels in all glass pressure gauges (33) and the water levels of the two water tanks in the video are collected at a time interval of Δt2; based on the buried depth of each glass pressure gauge (33) and the information collected by the video, a two-dimensional pore water flow field diagram of the middle sand tank at different moments is drawn, so as to obtain the variation law of the hydraulic exchange rate and flux of the undercurrent in the middle sand tank during the fluctuation of the water level in the left sand tank; S42: Based on the concentration data of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen collected from water samples, the distribution diagram of the three nitrogen concentrations in the pore water of the intermediate sand trough at the time interval of Δt2 and the ammonia nitrogen concentration penetration curve at different positions were drawn to reveal the influence of river water level fluctuations on the nitrogen migration range and conversion rate in the riverbank undercurrent zone.

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